1. Field of the Invention
The present invention relates generally to wireless communication systems, and, more particularly, to full-duplex wireless transmitters and receivers.
2. Description of the Prior Art
In the area of wireless communication systems, whose applications are varied and wide, typically half-duplex technology was and is currently employed. A half-duplex technology provides communication in both directions, but only one direction at a time (not simultaneously). Typically, once a party begins receiving a signal, it must wait for the transmitter to stop transmitting, before replying. Accordingly, half-duplex technology restricts the throughput that can be realized by using full-duplex technology.
Multi-input-multi-output (MIMO) systems, employing multiple antennas, are typically used for wireless applications, however, under severe shadowing environment, not only is throughput limited, the distance the signal can reach is also limited. For example, in home environments, because of room partitions and fewer windows, a transmitted signal cannot reach its destination if it has to travel far. Therefore, wireless relays are employed and serve as repeaters to extend the range of the signal. But because a single channel is used for communication, the relay reduces throughput by half.
A full-duplex technology allows communication in both directions, and, unlike half-duplex, allows this to happen simultaneously. Land-line telephone networks are full-duplex, since they allow both callers to speak and be heard at the same time. A good analogy for a full-duplex system would be a two-lane road with one lane for each direction.
Attempts at considering using full-duplex for wireless communications have stalled due to current full-duplex designs requirement for a sharp filter resulting in unbearable increased costs for use in wireless devices, such as smart phones and the like.
Further, current full-duplex considerations use the radio frequency (RF) signal to cancel “self interference”. Self interference is the constructive or destructive interference resulting from reception of echoes of the original signal, which has the undesirable effect of fading. It is expensive and challenging to cancel self interference at RF levels. A more extensive explanation of prior art techniques and limitations follows.
Just like Internet has been the major driving force of the prosperity of information technology, wireless has been the same force of the growth of communication industry over the past decade. But nothing is going to be more explosive than the marriage of Internet and wireless for growing trends into a so-called “wireless Internet tsunami”. Fashion leading wireless Internet products and services like Apple's iPhone and iPad, Google's Andriod devices, and Netflix's movie streaming from Internet are just the precursors of the wireless Internet tsunami.
To support such phenomenon growth, wireless industry also generates a large quantity of innovations, most noticeable 802.11n in indoor wireless Local Area Network (LAN) area and the coming 4G in outdoor wireless service area. The fundamental innovations of both 802.11n and 4G are all about exploring the capacity of multiple antennas at reflective wireless space, MIMO technology. While MIMO in 4G is still evolving and under-developed, MIMO in 802.11n has gone through fairly extensive field tests. It is now fair to say on one hand MIMO has created noticeable gains and merits on the other hand MIMO has failed to deliver some of its promised service, as discussed below. It is believed that similarly 4G cannot depend on MIMO technology alone to fulfill the demands of new wireless service.
In all sorts of applications and services, video streaming probably is one of the most demanding services to test the underlying Internet and wireless technologies. For example, one of the design targets of 802.11n has been described as “to deliver three HD video streaming around a whole house”, which turns out to be a commercial promising specification. The failure of 802.11n to achieve this promise can be supported by observing all kinds of wired technologies still used by service providers to deliver video streaming. If 802.11n has achieved the promise, wireless shall be much more convenient and the preferred technology to deploy video steaming around houses.
While MIMO technology of 802.11n has, to some extent, achieve its intended purpose, under some statistic model (that is under average), it is still quite easy to find a house where the video streaming target is not achieved in that the video streaming is not reachable. It is not unusual that some corner around a house even one HD video streaming cannot be achieved. To achieve the promise “to deliver three HD video streaming around whole houses”, wireless technology beyond MIMO needs to be further explored.
With the success of a company like Netflix, video steaming around a house has become a reality and the demand to support such service uniformly across all houses is expected to increasingly intensify.
Given the inadequacy of one wireless hop, which includes one pair of wireless access point (AP) and wireless client, it is natural to consider multiple wireless hops between AP and client. The simplest form of two-hop wireless is a half duplex wireless relay (or repeater, extender), which is used to relay received wireless signal. An example of a half-duplex wireless relay system, well known in the art, is shown in FIG. 1. In FIG. 1, the original signal and the relayed signal usually use the same RF band in a time sharing manner, which reduces the throughput by half, even though the range could be extended.
In order to remove the penalty of reducing the throughput by half, several challenges must be overcome. First, the RF band (or channel) used for the transmission of the original signal must be different than that used for the relayed signal, otherwise, multiple collision scenarios can happen between different forward and reverse links. Secondly, the self interference generated by the relay node must be removed. The same challenges exist for full duplex wireless systems, where the self interference originates from the transmitted signal to the received signal in the same RF band.
In the case where full duplex is used, in FIG. 1, the original signal and the relayed signal are transmitted and received at different RF bands. FIG. 2 shows a high level block diagram of a prior art full duplex wireless system including a transmitter and a receiver. The receiver receives an RF signal from an antenna and a RF canceller is used to cancel the self interference from the received RF signal by using an output of a splitter from the transmitter side of the system. The output of the RF canceller is then amplified, using a low noise amplifier (LNA). Next, the output of the LNA is mixed with a low frequency signal to generate a baseband signal from the RF signal and the output of the mixer is filtered using a low pass filter (LPF). Next, a voltage variable gain amplifier (VGA) and serves to amplify the gain of the received signal. Next, the output of the VGA is converted from analog form to digital form, using an analog-to-digital converter (ADC). The output of the ADC is processed by a baseband adaptive canceller, which sends its output to a demodulator (not shown in FIG. 2). The baseband adaptive canceller also receives as another one of its inputs, the output of the modulator.
On the transmitter side, a digital-to-analog converter (DAC) receives the output of the modulator and converts it to analog form. The output of the DAC is then amplified using the baseband amplifier and up to this point, the signal is at baseband. Next, a mixer converts the frequency of the signal at the output of the baseband amplifier to RF and passes it onto a power amplifier (PA), which amplifies the signal and sends it to the splitter. The splitter splits the output of the PA to provide one of the splitter outputs to the RF canceller in the receiver, as previously discussed, and to provide the other output to the antenna. Of particular noteworthiness is the cancellation of self interference at RF, which poses the problems discussed above.
A copy of the amplified RF signal, generated by the splitter in FIG. 2, is fed into the receive path for RF self interference cancellation. The receive path consists mainly of gain control and phase adjustment which are used to duplicate the self interference received from the transmitting antenna that is coupled back to the receiving antenna.
In full duplex wireless systems, because the signal is at a high frequency, linearity is difficult to maintain. Also, SNR is low, which also makes full duplex wireless systems difficult to achieve.
What is needed is a method and apparatus to cancel self interference when a transmitted and received signal use different (but usually nearby) RF band, thus the term generalized full duplex wireless.